The present invention relates to a gallium nitride compound-based semiconductor light emitting device and a process for producing a gallium nitride compound-based semiconductor thin film.
Gallium nitride compound-based semiconductors have been noted as semiconductor materials for visible-light emitting devices and high-temperature working electron devices. In these devices, structures formed by laminating semiconductor thin films are generally employed.
As a method for growing a gallium nitride compound-based semiconductor thin film, organometallic chemical vapor deposition is well known. This method comprises feeding organometallic compound gases, e.g. trimethylgallium (hereinafter referred to as TMG), trimethylaluminum (hereinafter referred to as TMA) and trimethylindium (hereinafter referred to as TMI), as starting gases of Group III elements, and ammonia, hydrazine or the like as a starting gas of a Group V element into a reaction tube having a substrate set therein, and maintaining the substrate temperature at a high temperature of approximately 900.degree. C. to 1,100.degree. C. to grow a gallium nitride compound-based semiconductor thin film on the substrate. When a gallium nitride compound-based semiconductor is grown by this method, a material different therefrom but relatively similar thereto in lattice constant, such as sapphire or silicon carbide (SiC) is used as the substrate because there is no substrate made of a gallium nitride compound-based semiconductor.
However, there is a large lattice mismatch between a gallium nitride compound-based semiconductor and a sapphire or SiC substrate, so that when the gallium nitride compound-based semiconductor is grown directly on the substrate at a high temperature, it grows into an island structure. Therefore, no flat surface can be obtained.
As a means for solving this problem, there is a method of, as disclosed in JP-A-2-229476 and JP-A-6-151962, growing a buffer layer of amorphous aluminum nitride (AlN) or gallium aluminum nitride (AlGaN) on a substrate at a low temperature before growing a gallium nitride compound-based semiconductor, and growing the gallium nitride compound-based semiconductor on the buffer layer at a high temperature. By this method, there have been produced light emitting devices having a structure formed by laminating gallium nitride compound-based semiconductor thin films.
However, even if the conventional method using the buffer layer grown at a low temperature is employed, the gallium nitride compound-based semiconductor thin film grown on the buffer layer has many defects. Therefore, the light emitting devices produced by using such thin films have no satisfactory characteristics. For realizing the production of a highly efficient light emitting device, there is desired further improvement in the crystallinity of gallium nitride compound-based semiconductor thin films.
The above-mentioned conventional technique uses the buffer layer grown at a low temperature before growing a gallium nitride compound-based semiconductor thin film at a high temperature. In such a method, in general, the quality of the gallium nitride compound-based semiconductor thin film to be grown on the buffer layer is improved only by optimizing, for example, a material and growth conditions for the buffer layer to be grown at a low temperature or growth conditions for the gallium nitride compound-based semiconductor to be grown at a high temperature, and no particular attention is paid to improvement of the quality of the gallium nitride compound-based semiconductor thin film which is carried out by noting the structure of the thin film itself in the vicinity of the boundary surface between the thin film and the buffer layer.
Therefore, the present invention is intended to provide a gallium nitride compound-based semiconductor light emitting device having excellent light-emitting properties.
The present invention is also intended to provide a gallium nitride compound-based semiconductor thin film excellent in crystallinity.
When AlN is used as the buffer layer, the growth of a two-dimensional and flat gallium nitride compound-based semiconductor thin film is macroscopically possible, but microscopically, the unevenness of the surface of the thin film remains and the crystallinity is not sufficient. Therefore, the thin film is desired to be improved. Particularly in a quantum well structure device comprising gallium nitride compound-based semiconductor thin films, the heterojunction boundary surface is required to have flatness at the atomic level. Conventional methods are disadvantageous in that no satisfactory light-emitting properties can be attained because the structure of a light-emitting layer located at the heterojunction boundary surface becomes nonuniform due to the unevenness of the surface of the thin film.
Accordingly, the present invention is intended to provide a gallium nitride compound-based semiconductor light emitting device having excellent light-emitting properties.
The present invention is also intended to provide a process for producing a gallium nitride compound-based semiconductor thin film excellent in surface flatness and crystallinity.